The present invention relates to a micromechanical part and a method for its manufacture.
The function of micromechanical parts may be impaired by adhesive forces that are active, when unfavorable conditions exist between facing surfaces of moving components of the parts. This adhesive tendency, also known as xe2x80x9cstiction,xe2x80x9d may be attributable to Van der Waals and capillary forces, electrostatic interaction, physical bonds and hydrogen bridge bonds between facing surfaces.
This adhesive phenomena may considerably impair the operational capability of micromechanical parts, such as, for example, sensors, such as acceleration or engine speed sensors. Such sensors may include a seismic mass, which is moveable against a surrounding frame under the influence of the acceleration or rotation to be detected, and the movement of which may be detected by verifying the change in capacity of capacitors, the plates of which are formed by facing surfaces of the seismic mass and the frame. Each adhesive force, which is active between facing surfaces of this type and causes the seismic mass to move, may result in severe corruption of the sensor measurement results.
To reduce solid-state adhesion, these surfaces may be chemically stabilized with masking layers, such as self-organizing monolayers, the surfaces of which are hardened by coating, for example, with adamantine carbon layers, or the surface topography of which, i.e., the shape of the contact surfaces and, for example, their surface roughness, is optimized. These methods may be costly, since the facing surfaces may not be easily accessible for subsequent processing. In addition, they may not always produce the desired results.
An exemplary micromechanical part according to the present invention and an exemplary method according to the present invention for its manufacture, which is believed to be simple and economical, may effectively reduce the adhesive phenomena.
An exemplary embodiment according to the present invention is based on corrupted measured values in micromechanical sensors varying gradually during the course of sensor operation, which may be attributed to the gradual formation of electrostatic charges on non-conductive sensor surfaces. To eliminate these electrostatic charges, a conductive coating may be provided, at least on facing surfaces of a micromechanical part, to enable the electrostatic charges to dissipate.
The conductivity of a coating of this type may be orders of magnitude lower than that of a typical electrical conductor of the micromechanical part, such as the supply conductor of a capacitor electrode. A low conductivity may be sufficient to dissipate minimal current intensities associated with electrostatic charges, without noticeably impairing the operation of the part.
To apply a coating having a nonvanishing, yet low, conductivity in a controlled manner, the coating may be created from a material that has a nonvanishing conductivity and does not form a highly insulating oxide on its surface. One suitable material is germanium, which may be applied in its pure form or, if necessary, with a dopant to control its conductivity.
If a substrate of the micromechanical part is made of silicon, a germanium coating may be easily structurable, since, while germanium nucleates on a silicon surface, it does not nucleate on a surface made of silicon oxide.
An object of an exemplary embodiment of the present invention is to provide a method for manufacturing a micromechanical part, having components that move with respect to one another, from a substrate, with a conductive coating being applied at least to facing surfaces of the components that move with respect to one another. A substrate of this type may suitably include a functional layer, which is structured, and an underlying sacrificial layer. The components that move with respect to one another may then be manufactured by etching trenches through the functional layer to the sacrificial layer and the removing of the sacrificial layer beneath at least one of the components.
The coating material is suitably selected, so that it accumulates on the functional layer, but not on the sacrificial layer. Applying the coating after etching the trenches, but before removing the sacrificial layer, permits the coating to be selectively applied, for example, to the side walls of the trenches, which, in a finished electromechanical part, form facing surfaces of the components that move with respect to one another, without requiring a separate masking step.
Germanium, for example, meets these requirements, if the functional layer is made of silicon and the sacrificial layer is made of silicon oxide.
Before the functional layer is structured by producing trenches therein, a masking layer is produced on part of the functional layer, and the trenches are formed by etching a portion of the functional layer not covered by the masking layer. Removing the masking layer only after the conductive coating is applied may prevent, depending on the type of masking layer, the coating material from being deposited on the masking layer or the deposited coating material from being removed with the masking layer. In this manner, an exterior of the substrate that was originally covered by the masking layer may be kept free of the conductive coating, in the finished part.
However, the masking layer may also be removed before applying the conductive coating, if a coating of this type on the external surface is desired or acceptable.
An LPCVD (low-pressure chemical vapor deposition) method may be used for depositing the coating, since this method may be performed at low temperatures, which avoids impairing a previously produced structure of the part.